This invention relates to methods of stripping extracted constituents from organic extractants. More particularly, this invention relates to methods of stripping constituents from quaternary amine extractants.
A number of important hydrometallurgical processes involve the extraction of metal species in the form of anionic metal complexes from aqueous pregnant liquors, and subsequent stripping of the metal values from the organic phase into an aqueous phase. Such extracting and stripping processes are especially important in the hydrometallurgy of Group V and Group VI metals. One particularly important source of aqueous pregnant liquors containing both Group V and Group VI metal values is the pregnant liquor produced from leaching spent hydroprocessing catalysts.
The chemistry of Group V and Group VI metals in aqueous solution is particularly complex. Each metal in both Groups forms a variety of oxidation states that can in turn form a variety of oxygen-containing anions in aqueous solution. These metal-oxygen complexes undergo complex reactions in aqueous solution that depend on such variables as pH, temperature, concentration and chemical environment. Each metal of the Group V and Group VI undergoes reactions at unique conditions. The complex aqueous chemistry of Group V and Group VI metals makes the liquid-liquid phase transfer processes particularly difficult, especially if more than one metal species of Group V and Group VI are present.
Crude oils frequently contain metals, particularly iron, nickel and vanadium. Spent hydroprocessing catalysts that have been in contact with metals containing crude or residual oil can therefore contain both molybdenum, a frequent catalytic metal, and vanadium. Extracting and stripping the vanadium values and the molybdenum values simultaneously from a leach liquor from spent catalysts therefore presents a difficult and subtle problem.
It is preferred, that if the metals from catalysts are to be recovered by hydrometallurgical techniques, that the total flow scheme be one that allows maximu recycle of reagents and minimum introduction of extraneous ions. Since one preferred recovery system involves an aqueous ammoniacal leach using ammonium carbonate, it is preferred that only ammonia, ammonium ion or carbonate ion be added to the aqueous streams. This self-imposed limitation complicates the recovery scheme of Group V and Group VI metals even more.
It is possible, at pH values in the range of about 3 to about 5, to extract both molybdenum and vanadium into an organic phase by use of a quaternary amine as a liquid anion exchange reagent. One such reagent is tricapryl methyl ammonium chloride, which is sold under the trade name Alaquat 336.RTM. by the Henkel Chemical Company.
Alaquat and related compounds that have extracted constituents from Group V and Group VI on them tend to be difficult to strip, especially if the strip is to be chemically compatible with the leach. In U.S. Pat. No. 3,083,085 an aqueous solution of NaCl is used to strip molybdenum from the organic phase. This requires regeneration of the organic extracting solution with sulfuric acid and the use of expensive alloys in process equipment that are not corroded by chloride ion.
U.S. Pat. No. 3,083,076 discloses the use of an aqueous solution of both sulfate and sodium hydroxide to strip molybdenum and vanadium from the organic solution. Again, this strip is chemically incompatible with the leach and requires corrosion resistant process equipment.
It has been discovered that an aqueous solution or bicarbonate anion is particularly effective for stripping both Group V and Group VI metals from organic solutions containing quaternary ammonium compounds. Bicarbonate provides high recoveries under mild conditions and is chemically compatible with the ammonical leach step in an overall process to recover metal values from spent hydroprocessing catalysts.